1. Field of the Invention
The present invention relates to an orthogonal spreading code for a CDMA mobile communication system, and more particularly, to a method for generating and allocating code pairs on the basis of an orthogonal code set, which is generated using orthogonal spreading codes, so as to reduce a peak-to-average power ratio and expand the length of an interference free window or IFW.
2. Description of the Related Art
In general, a Code Division Multiple Access (CDMA) mobile communication system adopts a spread spectrum communication method which uses a spread code with a transmission bandwidth which is much wider than that of an information signal to be transmitted. The spread spectrum communication method uses a wide frequency bandwidth, and thus can regenerate an original signal after despreading which increases the signal power and keeps the noise power low. According to a basic principle of the spread spectrum communication method, when a transmitting block modulates a data multiplied by a spread code to widen the bandwidth of a frequency and then transmits a signal, a receiving block multiplies the signal by the same spread code used in the transmitting block to narrow the bandwidth of the frequency and then demodulates the signal to detect the original signal. In general, the signal received through an antenna of the receiving block includes several kinds of noises mixed thereto in addition to the original signal. However, using the spread spectrum communication method converts the several kinds of noises into very weak electric power via a despreading process because the original signal is changed into a narrow bandwidth while the several kinds of noises are initially multiplied by the spread code to widen the bandwidth and remarkably reduce the interference of the noises when the receiving block multiplies the spread code for despreading.
The spread code used in such spreading and despreading processes can be used for spreading, synchronization and base station discrimination. In other words, autocorrelation and crosscorrelation processes can be executed for spreading, synchronization and base station discrimination. For detection of a desired signal, autocorrelation characteristics are required to have the maximum value when there are no time-offsets and a smaller value when time-offset values are not zero. Also, the crosscorrelation characteristics are required to have small values at all of the time-offsets for discrimination against a spread code used by another user.
In order to meet the foregoing autocorrelation and crosscorrelation characteristics, a conventional CDMA method uses a Pseudo Noise (PN) code together with a Walsh code as spread codes. The PN code satisfies required characteristics in autocorrelation, and the Walsh code satisfies required characteristics in crosscorrelation.
In the characteristics required in crosscorrelation single channel path allows no mutual interference among spreading codes allocated to a number of users. However, the mutual interferences exist among the spread codes having a number of channel paths. More particularly they are as follows:
With single channel path, the amount of mutual interference among the spread codes is determined only by the value of crosscorrelation having no time-offsets. On the contrary, with several channel paths, the amount of crosscorrelation among the spread codes is influenced not only by the crosscorrelation value having no time-offsets but also by the crosscorrelation values which have path delay times among separate channel paths as the time-offsets.
Therefore, in a multi-path channel environment which can be generally referred to as an actual channel environment, the crosscorrelation characteristics among the spreading codes are important not only in no time-offsets but also different time-offsets.
As a result, ideally the crosscorrelation values of the spread codes are required to be 0 at all of the time-offsets. However, it is not known so far about those codes for satisfying the crosscorrelation characteristics and the autocorrelation characteristics at the same time. In other words, referring to the PN and Walsh codes in use for the conventional CDMA method, the PN codes satisfy the required characteristics of autocorrelation while failing to satisfy the required characteristics of crosscorrelation. Also, the Walsh codes fail to meet the required characteristics of autocorrelation while only partially meeting the required characteristics of crosscorrelation. So, referring to the crosscorrelation characteristics of the Walsh codes, the crosscorrelation value is 0 when the time-offsets do not exist, but is not 0 when the time-offsets are not 0.
To solve such drawbacks, one of the orthogonal codes is proposed. The code is called Large Synchronization (LS) code. The LS codes perfectly meet the autocorrelation and crosscorrelation characteristics in a certain time-offset interval. The time-offset interval for perfectly meeting the autocorrelation and crosscorrelation characteristics will be defined as an Interference Free Window (IFW)
Referring to autocorrelation characteristics in the IFW, the autocorrelation value is the maximum where no time-offsets exist, and 0 at any time-offsets in the IFW where the time-offsets are not 0. Also, according to the crosscorrelation characteristics of the LS codes, the crosscorrelation value is 0 at any time-offsets in the IFW.
As a result, in the multi-path channel environments where the path delay time-offsets among the channel paths exist in the IFW, the interference among the spreading codes allocated to users can be removed. Therefore, the time-offset interval satisfying the foregoing autocorrelation and crosscorrelation characteristics is referred to as the Interference Free Window or IFW.
Conventionally, the PN codes and the Walsh codes partially satisfy the characteristics required in autocorrelation and crosscorrelation at the time-offsets in the IFW, whereas the orthogonal spreading codes perfectly satisfy the characteristics required in autocorrelation and crosscorrelation at the time-offsets in the IFW.
Although the orthogonal spreading codes have the autocorrelation and crosscorrelation characteristics excellent at the time-offsets in the IFW, however, there is a drawback that only a small number of codes are available in practice: i.e., the number of available orthogonal spreading codes decreases as the length of the IFW interval increase.
When a set of the orthogonal spreading codes satisfying the foregoing autocorrelation and crosscorrelation characteristics is defined as an orthogonal code set, the length of the IFW interval is inversely proportional to the number of elements in the orthogonal code set. When the length of the IFW interval increases as set forth above, the interference is proportionally reduced. However, the available number of orthogonal codes is restricted, thereby resulting in restriction of channel capacity.
A number of spreading methods using the orthogonal spreading, codes have been proposed, and examples thereof include: a Binary Phase Shift Keying (BPSK) spreading method in which the same orthogonal spreading code is used in both I branch and Q branch (FIG. 1), a Quadrature Phase Shift Keying (QPSK) spreading method in which different orthogonal spreading codes are respectively allocated to the I branch and the Q branch (FIG. 2), and a complex spreading method for reducing the power imbalance between the I and Q branch (FIG. 3).
However, if different orthogonal spreading are respectively allocated to the I branch and the Q branch in carrying out the spreading methods, the spreading codes of the I branch and the spreading codes of the Q branch can be simultaneously varied, and thus the spread signals may undergo a phase transition of 180 degree. Such a phase transition of 180 degree has a negative influence on the envelope fluctuation of the spreaded signals after passing through a filter, thereby increasing a Peak-to-Average Power Ratio (PAPR).